Isoprenoid is a general term for compounds having isoprene unit consisting of 5 carbon atoms as a backbone structure. Isoprenoid is biosynthesized by polymerization of isopentenyl pyrophosphate (IPP). Various kinds of isoprenoid compounds are present in nature and many of them are useful for humans.
For example, ubiquinone plays an important role in vivo as an essential component of the electron transport system. The demand for ubiquinone is increasing not only as a pharmaceutical effective against cardiac diseases, but also as a health food in Western countries.
Vitamin K, an important vitamin involved in the blood coagulation system, is utilized as a hemostatic agent. Recently it has been suggested that vitamin K is involved in osteo-metabolism, and is expected to be applied to the treatment of osteoporosis. Phylloquinone and menaquinone have been approved as pharmaceuticals.
In addition, ubiquinone and vitamin K are effective in inhibiting barnacles from clinging to objects, and so would make an excellent additive to paint products to prevent barnacles from clinging.
Further, compounds called carotenoids having an isoprene backbone consisting of 40 carbon atoms have antioxidant effect. Carotenoids such as β-carotene, astaxanthin, and cryptoxanthin are expected to possess cancer preventing and immunopotentiating activity.
As described above, isoprenoid compounds include many effective substances. Establishment of an economical process for producing these substances will be a huge benefit to the medical world and society.
The process for producing isoprenoid compounds through fermentation has already been examined, and examination of culture conditions, strain breeding by mutagenesis, and improvement of yield by genetic engineering tehniques have been tested. However, the practical results are limited to individual types of compounds, and there is no known method effective for the isoprenoid compounds in general.
Isopentenyl pyrophosphate (IPP), a backbone unit of isoprenoid compounds, has been proved to be biosynthesized from acetyl-CoA via mevalonic acid (mevalonate pathway) in eukaryotes, such as an animal and yeast.
3-Hydroxy-3-methylglutaryl-CoA (HMG-COA) reductase is considered to be a rate-limiting enzyme in the mevalonate pathway [Mol. Biol. Cell, 5, 655 (1994)]. A test in yeast to improve the yield of carotenoids by overexpression of HMG-CoA reductase has been conducted [Misawa, et al., Summaries of Lectures on Carotenoids, 1997].
There is no knowledge which proves the presence of the mevalonate pathway in prokaryotes. In many prokaryotes, another pathway, the non-mevalonate pathway, has been found in which IPP is biosynthesized via 1-deoxy-D-xylulose 5-phosphate produced by condensation of pyruvic acid and glyceraldehyde 3-phosphate [Biochem. J., 295, 517(1993)]. It is suggested that 1-deoxy-D-xylulose5-phosphate is converted to IPP via 2-C-methyl-D-erythritol 4-phosphate in an experiment using 13C-labelled substrate [Tetrahedron Lett 38, 4769 (1997)].
In Escherichia coli, a gene encoding an enzyme, 1-deoxy-D-xylulose 5-phosphate synthase (DXS) which allows biosynthesis of 1-deoxy-D-xylulose 5-phosphate by condensation of pyruvic acid and glyceraldehyde 3-phosphate, is identified [Proc. Natl. Acad. Sci. USA, 94, 12857 (1997)]. Said gene is contained in an operon consisting of four ORFs that include ispA encoding farnesyl pyrophosphate synthase.
Further in Escherichia coli, the presence of the activity to convert 1-deoxy-D-xylulose 5-phosphate to 2-C-methyl-D-erythritol 4-phosphate is known [Tetrahedron Lett 39, 4509 (1998)].
At present there are no known description nor suggestion to improve yield of an isoprenoid compound by genetically engineering these genes contained in the operon.
Although knowledge about the non-mevalonate pathway in prokaryotes has gradually increased, most enzymes involved therein and genes encoding these enzymes still remain unknown.
In photosynthetic bacteria, there is a known process for effectively producing ubiquinone-10 by introducing a gene for an enzyme ubiC (uviC gene), which converts chorismate into 4-hydroxybenzoate, and a gene for p-hydroxybenzoate transferase (ubiA) (Japanese Unexamined Patent Application 107789/96). However, there is no example which improved the productivity of isoprenoid compounds by genetically engineering genes for enzymes involved in the non-mevalonate pathway.
Moreover, there is no knowledge about how prokaryotes will be influenced when the reaction on the non-mevalonate pathway is inhibited by mutagenesis or treating with drugs.